A. Field of the Invention
The present invention relates generally to optical waveguide couplers and more specifically to optical couplers using single-mode optical fibers that are resistant to polarization changes.
B. Description of the Prior Art
Fiber optic couplers are commonly used to transfer light from one optical waveguide to another for a variety of functions. For example, fiber optic couplers are used in the communications field to rapidly convey larger amounts of data. Other applications include measurement systems in which high-precision measurements are made based upon an environmentally dependent shift in either the phase or wavelength of light traveling through an optical waveguide. Early couplers used for measurement applications had several disadvantages. For instance, the coupling ratio of early couplers typically varied with the polarization of the light passing through the coupler. Furthermore in some cases, the phase of the light passing through the coupler varied with polarization which was unsatisfactory for certain interferometric sensor aplications such as the passive quadrature demodulator coupler matrix.
Much of the technology developed in the past ten years for manufacturing couplers has centered around couplers which use multimode fibers. Multimode optical fibers are fairly large fibers, having a core diameter on the order of 40 microns to 200 microns. Some multimode fibers may be as large as 4 millimeters. While multimode fibers can be used to produce couplers for transferring polarized light from one fiber to another, they are typically undesirable because polarized light transmitted in a multimode optical fiber has a tendency to become depolarized. Single-mode fibers, on the other hand, are more capable of transmitting polarized light with minimal depolarization of light during transmission over small distances. Due to the method in which single-mode fibers act to transmit only one mode of light, the core diameter of a single-mode fiber is approximately ten times smaller than the core diameter of a multimode fiber. This has led to serious difficulties in manufacturing couplers using single-mode fibers. The invention described hereinbelow represents an advancement in the art in that it teaches methods for coupling single-mode fibers and for producing polarization-insensitive optical couplers.
Before describing the applicants' invention however, it is necessary to briefly describe the physical characteristics of a typical optical fiber. In most instances, a fiber comprises an inner core having a relatively high index of refraction, a cladding having a relatively low index of refraction, and a substrate having a relatively high index of refraction, but not necessarily equal to the index of refraction of the core. This type of fiber is commonly called "W-fiber" by those skilled in the art because the index profile of the fiber appears to resemble a "W". It should be noted that other types of fibers do not have an outer substrate having a relatively high index of refraction.